Agriculture Reference
In-Depth Information
Nutrients and Leaf Longevity
The decrease in leaf longevity with higher levels of foliar nitrogen content is a
well-established interspecific relationship (Field and Mooney 1986; Field
1991; Reich et al. 1991, 1992, 1994; Wright et al. 2004; Poorter and Bongers
2006), but this negative relationship may or may not apply within species or
among species at a site. Observational and experimental evidence for the effect
of fertility on leaf longevity in general shows that for a given species leaf lon-
gevity will be shorter at more fertile sites. For example, leaf longevities of
Picea abies
,
P. jezoensis
, and
P. glehnii
were longer on nutrient-poor serpentine
soil compared to more fertile brown forest soil (Kayama et al. 2002).
Fertilization of the prostrate tundra evergreen shrub
Ledum palustre
var.
decum-
bens
increased leaf turnover (Shaver 1981). Fertilization of
Pseudotsuga men-
ziesii
var.
glauca
and
Abies grandis
, coniferous trees of the Pacific Northwest
in North America, reduced leaf longevity by about one-fourth (Balster and
Marshall 2000). In the Hawaiian tree
Metrosideros polymorpha
, leaf longevity
varies between 2 and 5 years and is longer on more infertile sites; fertilization
decreases longevity on fertile sites but not at the infertile sites where longevity
is already long (Herbert and Fownes 1999; Cordell et al. 2001). In this tree spe-
cies longevity decreased as leaf nitrogen content increased across sites (Herbert
and Fownes 1999). In
Larrea tridentata
, an evergreen desert shrub, fertilization
shortened leaf longevity, and the effect was enhanced by irrigation (Lajtha and
Whitford 1989; Fig.
8.4
). A 100-fold increase in nutrient availability decreased
leaf longevity of the perennial floating-leaved aquatic plant,
Hydrocharis
morus-ranae
var.
asiatica
, from 15-20 to 10-15 days (Tsuchiya 1989); lower
levels of fertilization did not significantly alter leaf longevity (Tsuchiya 1989;
Tsuchiya and Iwakuma 1993).
Box 8.2
Density Dependence
A density dependence in population regulation occurs whenever differ-
ences in either birth rate or death rate result in lowering of the population
growth rate as the density of the population increases. If the density depen-
dence is driven by changes in the death rate of individuals, we speak of
density-dependent mortality factors. In general a population is considered
to be regulated at some equilibrium density by density-dependent factors
such as the reduction of birth rate resulting from short supply of food,
increase in death rate from overcrowding, and similar regulatory responses.
Without some sort of density-dependent factors, population numbers could
not be regulated.
